Deflecting device for cathode ray tubes



Jan. 28, 1941, J. GUNTHER DEFLECTING DEVICE FOR CATHODERAY TUBES FiledNOV. 22, 1959 FIG. I.

INVENTOF? ATTORNEY Patented Jan. 28, 1941 UNITED STATES DEFLECT'INGDEVICE FOR CATHODE RAY TUBES Johannes Giinther, Berlinezehlendorf,Germany,

assignor to Fernseh Aktiengesellschaft, Berlin- Zehlendorf, GermanyApplication November 22, 1939, Serial No. 305,696 In Germany November30, 1938 3 Claims.

The present invention relates to deflecting means of the electromagnetictype for cathode ray tubes. Cathode ray tubes are used for televisionpurposes as well as oscillograph purposes and in both cases it .is'desired to trace upon the screen a scanning pattern of substantiallyrectangular form. Tubes of this type, particularly those having a wideangle of deflection and relatively short length, show distortions at theedges of the scanned area consisting for instance in a pincushion orbarrel shaped outline of the area swept out on the screen. This happensparticularly in case the radius of curvature of the screen surface islarger than the length of the deflected ray.

It is an object of the present invention to avoid distortions of thistype in cathode ray tubes. It is a further object to provide adeflectingdevice consisting of two deflecting systems for twocoordinates of the scanned pattern, one of said systems having the formof coils arranged on both sides of the neck of the tube and having smalldimensions, in particular a short length.

According to the invention the two deflecting systems are so arrangedwith regard to one another that the cathode ray deflected by the firstsystem passes near the front leads of the coils of the second deflectingsystem when the edge portions of the screen are scanned. The frontconductors of the coil of the second deflecting system produce amagnetic field which counteracts to a certain degree the main field ofthe coils. This oppositely directed field is made use of to produce acompensation of the distortions. The

" cathode ray is deflected by the first system in such a manner that itpasses through the magnetic field set up by the front conductors of thecoil on the side of the coils which is directed towards the screen sothat the deflection is more or less weakened as the ray enters thecounterfleld to a greater or smaller degree.

The drawing shows an embodiment of the invention by way of example.

Fig. 1 is a side view of a cathode ray tube carrying two deflectingcoils and a deflecting yoke.

Fig, 2 is a view of the coils with a representation of the lines offorce.

Fig. 3 is a cross section through the tube in the plane of the line A-A.

Fig. 1 shows the cathode ray tube l with a screen 2 and a neck 3.Deflecting coils 4 are arranged upon the neck of the tube. Anotherdeflecting system 5 consisting of an iron yoke is arranged in such amanner that the field set up between the poles of the yoke liesrectangular to the field produced by the coils.

The field produced by the coils 4 is represented in Fig; 2. This figureshows the field distribution in a plane being through the central axisof the tube. The main deflecting field is indicated by arrows 6 and thelines of force are closed around the side conductors of the coils asindicated in Fig. 3. The front conductors l and the rear conductors 8 ofthe coils produce also a magnetic field as indicated by arrows 9. Thismagnetic field increases the main field on the inner side of the coilbut it is opposed in direction on the outer side of the coil. The linesof force shown in the drawing exist for a particular value of deflectingcurrent and are reversed when the current is reversed in direction.

The cathode ray indicated by line passes through the central axis of thetube and is deflected in such a manner that it leaves the space betweenthe coils with an angle depending upon? the current flowing through thecoils. The first deflecting system consisting of the yoke is so arrangedwith regard to the coils that the ray does not pass through the fieldset up by the rear conductors of the coil but through the stray field ofthe front conductors. The lines of force lying on the outside of thecoil act upon the deflected cathode ray more strongly than the lines offorce lying on the inside so that the deflection is weakened in thoseparts where the cathode ray approaches most closely the front conductor.This can be seen from Fig. 3 in which the area swept by the cathode rayin the plane of the front conductors is indicated by dotted lines II. Ifthe distortion produced by the particular shape of the screen wouldresult in a pin-cushion outline of the scanned area it can be seen fromthis figure that the deflection is weakened near the corners of therectangle H so that the pincushion distortion is compensated. Thestrength of this compensating effect can be varied in dependency uponhow far the cathode ray enters the region of the stray field, and howstrong the inhomogeneous stray field is. It has been found that thecompensating effect increases when the curvature of the deflecting coilis increased, i. e. when the coils extend farther around the tube.

The same basic idea can be used for compensating a barrel shapeddistortion. In this case the coils must be curved in the oppositedirection, for instance, convexly to the cathode ray tube.

The dimensions, particularly the length of the coil, is a means forobtaining the desired strength of the counterfield. It is possible forinstance by means of particularly short coils to obtain a stronginfluence upon the cathode ray because the front conductors produce inthis case a counterfield, which is comparatively strong in comparison tothe main field. It is preferable in this case to make the coils only alittle longer than the diameter of the neck of the tube, for instance,1.4 to 1.8 of the tube diameter.

Another feature of the invention is to arrange the coils 4 closely tothe point where the funnel shaped part of the tube begins and to use atube of wide opening so that the ray can be deflected very stronglywithout touching the wall of the tube.

The first deflecting system may have a construction of any suitable kindand may consist also of coils instead of the yoke mentioned above or maybe an electrostatic deflecting system. The invention may be used forcompensating distortion in one or two directionsof deflecting.

The invention is suited particularly for television transmission andreceiving tubes but it can also be used in connection with other typesof cathode ray tubes showing not only pin-cushion or barrel shapeddistortions but edge distortions of any kind. It is also possible tocompensate distortions which are not due to the curvature of the screenof the tube but which are caused by an optical system used in connectionwith the cathode ray tube, for instance, for projection purposes.

What I claim is:

1. In combination with a cathode ray tube having a screen adapted to bescanned by a beam of electrons, a first deflecting system for developinga first magnetic deflecting field of varying intensity and polarity in afirst direction perpendicular to the axis of said tube and forsimultaneously developing magnetic counterfields varying in accordancewith said first field in radial directions with respect to said tubeaxis, and a second 'deflecting system for developing a second magneticdeflecting field in a second direction perpendicular to the axis of saidtube of varying polarity and such intensity periodically to deflect saidbeam of electrons into one of said counterfields at extreme angles ofdeflection, thereby to influence by said counterfields the beamdeflection effected by said second field.

2. In combination with a cathode ray tube having a screen adapted to bescanned by a beam of electrons, a first deflecting system including sideconductors for developing a first magnetic deflecting field of varyingintensity and polarity in a first direction perpendicular to the axis ofsaid tube and including end conductors for developing magneticcounterfields varying in accordance with said first field in radialdirections with respect to the axis of said tube, and a seconddeflecting system for developing a second magnetic deflecting field in asecond direction perpendicular to the axis of said tube of varyingpolarity and such intensity periodically to deflect said beam ofelectrons into one of said counterfields at extreme angles ofdeflection, thereby to influence by said counterfields the beamdeflection effected by said second field.

3. In combination with a cathode ray tube having a neck portion, a bulbportion, and a screen positioned in said bulb portion adapted to bescanned by a beam of electrons, a first deflecting system comprisingcoils having side conductors for developing a first magnetic deflectingfield of varying intensity and polarity in a first directionperpendicular to the axis of said tube and having front end conductorsfacing said screen and rear end conductors remote from said screen forsimultaneously developing magnetic counterfields varying inaccordance-with said first field in radial directions with respect tosaid tube axis, and a second deflecting system for developing a secondmagnetic deflecting field in a second direction perpendicular to theaxis of said tube of varying polarity and such intensity periodically todeflect said beam of electrons into one of said counterfields developedby said frontend conductors at extreme angles of deflection,

thereby to influence by said counterfields the beam deflection effectedby said second field.

JOHANNES GiiNTHER.

